The present invention relates to liquid level monitoring and, in particular, to monitoring oil levels in gas turbine engines. Gas turbine engines include lubrication systems that use a lubricating liquid, such as oil, to cool and lubricate components such as gears and bearings. Such systems typically use oil during operation such that the quantity of oil in the lubrication system is diminished over time. Thus, lubrication systems typically include one or more sensors to determine the quantity of lubrication in the system at any given time.
It can be difficult, however, to accurately determine the quantity of oil in a lubrication system at a given time. For example, one type of gas turbine engine is an auxiliary power unit (APU) for use on an aircraft. Varying oil temperature causes the actual volume of oil to vary, making it difficult to obtain useful sensor measurements. Sloshing of oil during aircraft movement also increases the difficulty of useful measurements. Consequently, oil levels reported to a pilot can be inaccurate, causing the pilot to believe that he needs to add oil (when he actually does not) or causing the pilot to believe that the system has sufficient oil (when it actually does not). Systems designed to account for such measurement difficulties can be complicated, expensive, and unreliable.